The subject matter disclosed herein relates to a set of coaxial counter-rotating rotor assemblies in which each individual blade is controlled separately and, in particular, to controlling rotor blades of the rotor assemblies to receive different rotor pitch angle control commands in different rotor azimuth angle.
Helicopters using coaxial counter-rotating rotors use differential collective between the rotors to provide yaw control at low and moderate speeds. The differential collective makes the blade collective angle of one rotor go up and the blade collective angle of the counter-rotating rotor blade go down. This provides increased torque for one rotor and decreased torque for the other rotor. Since the rotors rotate in opposite direction the net yawing moment felt by fuselage add up in the same direction.
Rotor blades of rotary-wing aircraft, such as helicopters, typically receive a down-flow of air coming to the rotor from above. In this conventional case the differential collective provides a yawing moment in an expected, and correct, direction. When the rotors receive an up-flow of air coming to the rotor from below, the torque, and therefore yawing moment, created by increasing blade collective may be in the opposite direction, and control of the aircraft is degraded. This is called autorotation state of the rotor. In this case the rotor is producing lift but requires no power applied to the rotor because up-flow through the rotor produces torque in the direction to propel the rotor and balances the torque required to rotate the rotor. At forward speeds the down-flow (in-flow) of the rotor is reduced and even changes to up-flow on the forward part of the rotor and the down-flow is increased on the aft part of the rotor.
Additionally, as the aircraft descends, the rotor receives air from beneath the rotor and up-flow increases. Therefore the yaw control provided by differential collective in co-axial counter rotating rotors degrades significantly at forward speeds and descending flights. Additional means of yaw control are provided in such designs. These include large rudders located at the back end of the helicopters. Because rudder effectiveness is proportional to the square of helicopter speed, large rudders are necessary to be useful at lower speeds. In addition, thrusters may be used. Both these designs increase cost, weight and complexity of the aircraft.